As a positive active material for a nonaqueous electrolyte secondary battery, a lithium transition metal composite oxide such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2) which has a lamellar rock salt type structure, or lithium manganese oxide (LiMn2O4) which has a spinel structure has been used. In particular, the lithium manganese oxide which has a spinel structure (hereinafter, referred to as a spinel type lithium manganese oxide) is high in thermal stability of a crystal structure, and a battery in which the lithium manganese oxide is used in a positive electrode has been widely employed because of exhibiting a high safety even at the time of being abnormally heated.
However, the theoretical capacity of the spinel type lithium manganese oxide is 148 mAh/g and is smaller than that of a lithium transition metal composite oxide which has a lamellar rock salt type structure (hereinafter, referred to as a lamellar type lithium transition metal oxide), and there is a problem that the energy density of a battery is decreased. Moreover, a problem that the power of a battery in which the spinel type lithium manganese oxide is used is lower than that of a battery in which the lamellar type lithium transition metal oxide is used has been caused.
In contrast, there has been proposed a method of mixing a lamellar type lithium transition metal oxide and a spinel type lithium transition metal oxide to be used for the positive active material. For example, in Patent Document 1, there has been proposed a lithium secondary battery in which an enhancement in energy density is achieved by mixing a lamellar type lithium transition metal oxide containing nickel, cobalt and manganese and a spinel type lithium manganese oxide to provide the positive active material. Moreover, in Patent Document 2, there has been proposed a lithium secondary battery in which highly enhanced output and prolonged life of the battery are achieved by allowing a lamellar type lithium transition metal composite oxide containing manganese and nickel and a spinel type lithium manganese oxide to be contained in the positive active material and allowing the composition ratio of nickel to the transition metal element other than lithium in the lamellar type lithium transition metal composite oxide to be 50% or more in terms of the molar ratio.